Although the use of the electron microscope (EM) by the Laboratory of Infectious Diseases (LID) reached its peak activity in the 1970's with the discovery of the Norwalk virus, Hawaii virus, Montgomery County virus, and hepatitis A virus, as well as the initial detection in the United States of human rotavirus (which was discovered in Australia), there has been a marked resurgence of usage of the EM recently. This has resulted from the expression of recombinant 27nm virus-like particles bearing the outer capsid antigens of Norwalk virus (by others) and Desert Shield, Hawaii, and Toronto viruses (by LID scientists)--all associated with acute gastroenteritis and all members of the family Caliciviridae. The utilization of the EM has proved to be essential because none of these fastidious viruses has yet been propagated successfully in cell culture. Therefore, the EM remains as: (i) the only method for the detection of the 27nm recombinant virus-like particles as well as the native 27nm virus particles, and (ii) the only method for the demonstration of specific antigenic relationships among these recombinants (by immune electron microscopy). In addition, the EM has been an important adjunct to many of the projects of the section providing seminal information in the progress of molecular biologic (as well as other) studies. The scope of the use of the EM is supported by the fact that approximately ninety individual experiments were conducted by electron microscopy since the previous annual report.